The invention relates to a method of heat integration and optimum configuration for a single or multi pressure level Supercritical Steam Heat Recovery Steam Generator (HRSG) with reheat used in combined cycle applications.
The most commonly applied steam bottoming cycles for combined cycle power plants currently being installed and operated are reheat steam cycles with sub-critical steam generation at multiple pressures and a single reheater located in the heat recovery steam generator gas path upstream of the high pressure evaporator. This combined cycle configuration was first described in a paper, “GE MS7001FCombined-Cycle Power Plant” by Leroy O. Tomlinson, Roger O. Anderson and Raub W. Smith that was presented to the American Power Conference in April, 1987 and published in the proceedings of this conference. The current state of the art irreversibility losses are ˜10% of GT exhaust energy.
Performance of combined cycles with the reheat, sub-critical steam cycle with steam generation at multiple pressures can be improved by incorporating a two-stage reheater as described in U.S. Pat. No. 6,220,013 B1, Multiple Pressure Reheat Combined Cycle with Multiple Reheaters by Raub W. Smith.
Supercritical steam cycles have been designed for combined cycles with a single reheater in the heat recovery steam generator gas path upstream of the steam generating section as described in the article entitled “Going Supercritical—Once-Through is the Key” published in Modern Power Systems, December, 1998.
The performance of the current bottoming cycle technology (mostly sub-critical pressure) is limited by the pinch points which occur in the HRSG evaporators due to constant temperature phase change from water to steam, with heat equal to the latent heat of vaporization required for this process. This discontinuity in temperature causes a mismatch between the gas turbine exhaust gas and the water/steam heating, resulting in significant irreversibility in the cycle.
The fundamental advantage of supercritical combine cycles originates from the physics of fluids at supercritical conditions. Water above the supercritical pressure behaves in a different manner when heated. At supercritical conditions, the water temperature in the boiler increases continuously without discontinuities due to phase change. This behavior allows for better matching of the gas turbine exhaust gases with the water/steam for less irreversibility during energy transfer. This behavior and benefit has been known in the past, but the performance gains were not big enough to justify the additional cost in combined cycle applications.